This invention relates generally to the sorting of pellets manufactured in a preforming press, and more particularly this invention relates to a method of and a device for the sorting of pellets molded in the preforming press having a rotary die disk, a punching station and means for measuring the compressing force applied on the pellets during the molding process, and means for selectively guiding the completed pellets off the disk in response to an actuation signal from the measuring means.
Preforming presses are known in which the manufacturing of pellets takes place in dies arranged around the periphery of a rotary die disk and in which a powdered material to be molded is compressed by punches. In order to preserve a predetermined quality of all pellets during the molding processes, the forces applied by the punches in respective dies are continuously measured and compared with nominal values. If the ascertained compression force applied against a pellet is above or below a range defined by two limit nominal values, the resulting pellet has to be discarded. For this purpose, an ejecting device is used which is located downstream of the punching station and is selectively actuated in response to the approach of a bad or good pellet. The control of this ejecting device in prior-art presses is effected by means of signals which upon actuation of a first punch proximity switch are generated by a compression force monitoring unit and fed into a shift register.
Such prior-art methods and devices, however, have the disadvantage that they can operate only at a very low sorting frequency which is unsuitable for high speed preforming presses. The term "sorting frequency" means the number of possible sorting processes carried out by the device in a time unit such as a second, for example. This low sorting frequency results principally from the fact that in prior-art sorting devices there is no possibility to shift the time point of actuation of the sorting elements in response to the manufacturing rate or speed of the preforming press.
Conventional sorting devices operate at a sorting frequency of about 30 Hertz (Hz), which means 30 possible working cycles per second. In high performance preforming presses of recent design, however, it is possible to manufacture as many as about 80 pellets per second.